Bachem Natter on mobile launcher

[Weaver]

The shortest steam catapult that Wikipedia shows is 162 feet (49 m) long (Waist catapults on USS Midway and USS Franklin D. Roosevelt)
The longest is 325 feet (99 m) .
https://en.wikipedia.org/wiki/Aircraft_catapult#Steam_catapults_types

You'd need quite a land vehicle for the length and the forces involved.

HMS Centaur had 139 foot cats and there may have been smaller ones. Still too large for a land vehicle though, and also, where do you get the steam from?

[Rheged]

Have we considered a steam locomotive hauling a sectional catapult that can be locked into a single unit on a straight length of track?  Steam at 250 psi from a normal loco, 450psi from one of Nigel Gresley's specials.  This may be a non-starter everywhere but in Whiffworld but it's different!

[Wardukw]

Well you wouldn't need anything as huge as a steam catapult..the 163 weights a bit over 4.2tons max take off weight which isn't that heavy but I've had a idea to slove the GeForce issue.
Using a JATO type system with a progressive feed system for the JATO the pilot wouldn't feel that many geforces.
The way it works is once fired the Jato doesn't dump all its power in one hit..it releases it slower and being timed for maximum power which comes on say 4 seconds later by which time the plane is half way up the launch rail and when full power is released the plane is already moving and it's main rocket engine can ignite giving it full power to everything.
The geforces should be a hell of a lot less ..will it work ? ..can't see why not and with the desperate need for anything to stop the bombers the Germans would try anything and in the RW we all know they pretty much did ..ie the Natter.
An extendable launch rail would be a must just for that little bit of extra safety and a better chance of success.
A 12 ton half track would work ..a 18tonner would to but that is to bloody expensive to buy and I don't have one